Add LoadTextureFromTexture function

description

The D3DX11LoadTextureFromTexture can be used to copy from one D3D11 texture to another starting with a specific source/dest box/rect, mip level, array element, num elements. It performs necessary conversions.

DirectXTex can be used to implement something like this using CopyRectangle, but it would be useful and informative to have a more full-featured LoadTextureFromTexture in the DirectXTex library.